Communications between digital devices over various transmission means, such as telephone lines and microwaves, require the use of modems to convert the digital signals from the device into a modulated signal of the type appropriate for the transmission medium. The devices with which applicant is familiar have in the past always been designed to use the same type modem on all output ports. If another modem type requiring a different command set were to be used, a different board would be inserted into the device to interface with the new modem types. It would, of course, be beneficial to allow any type of modem to be connected to any given port on a digital device without changing out printed circuit boards and being able to take advantage of the physical availability of different types of modems as they occur. An obvious solution to such a situation would be to install some type of switch at each output port of the digital device, which could be set to a setting corresponding with a command set for each of several different modems. The digital device could then check the switch setting and use commands appropriate to the setting. However, such a solution would still be subject to technician errors or omissions in getting the setting right and, further, would not take care of the possibility that, instead of a wrong switch setting, a modem is inoperative or removed.
The present invention attacks the problem by keeping a list of all modem types, to be used, within storage means of the digital device and the associated command sets used for each type. Further, a unique initialization command for each modem type is kept in storage whereby the modem can be interrogated and the expected response checked for each of the modem types in succession until it is accurately known which modem type is connected. Further, if the commands issued result in no response or prescribed given response, it can be determined whether the modem is inoperative or is not connected and appropriate error signals activated.
It is, therefore, an object of the present invention to provide an improved interface between a digital device and modems to be connected thereto, whereby a dynamic modem selection algorithm is employed to logically ascertain the modem type and thus, the selection of appropriate command sets to be used with the modem.